For magnetic disk recording the end goal is an ability to write and read either 0 (represented by a low state) or 1 (represented by a high state). Till now, only analog output type reader designs have been implemented. To function properly, such devices need to keep the free layer longitudinally biased which result in low sensor utilization rate (defined as: output voltage/(bias voltage times full dR/R)). A device that was truly digital (as opposed to analog) would make more efficient use of the sensor's capabilities, thereby solving many of the problems currently associated with device dimension shrinking.
In FIG. 1 we show an ABS (air bearing surface) view of a typical current perpendicular to plane (CPP) read element. The sensor has top and bottom shields 11 and 12, respectively, and is patterned to have the required reader width (long dimension of free layer 14). Also shown are seed layer 16, pinning layer(s) 17, pinned layer(s) 18, and capping layer 19. Note the presence of hard magnets 13 that are magnetized in the X-direction, thereby magnetically biasing free layer 14 in its longitudinal direction. Depending on the nature of spacer layer 15, the sensor can be a tunneling magneto-resistive (TMR) type, including a current confined path (CCP), if layer 15 is insulating, or it could be a metallic giant magneto-resistive (GMR) read head if layer 15 is conductive. The remaining space is filled with insulation 20.
The stripe height (SH), as measured in the z-direction (normal to the plane of the figure), of the sensor is usually 0.9 to 1.3 times the reader width, depending on film properties, the particular reader width, hard bias strength, and device MRR (magneto-resistance ratio), etc. The goal is to achieve acceptable stability for free layer (FL) 14 through the provision of the proper bias. The utilization rate of this type of design ranges from 20-35% when used in conjunction with perpendicular magnetic recording (PMR).
In FIG. 2 we show curve 25 which is a plot of output voltage (in μV) as a function of the strength of the magnetic field which it is sensing, from −600 to 600 Oe, for a device of the prior art, said device having been given a longitudinal bias of about 1000 Oe. Without the presence of the latter, the central portion of curve 15 would lose its linearity, becoming non-reproducible and noisy, often changing every time the magnetization was reversed. The resistance R of the device is ˜373% and the applied voltage is 140 mV. The total output voltage is ˜20.4 mV corresponding to a total resistance change of ˜54.4Ω and dR/R˜14.6%. Since the optimum dR/R of the device is ˜50%, its present utilization rate is ˜29%.
The current generation of device readers has a width of about 70-100 nm. With high amplitude MgO films, reasonable performance can be achieved for area densities up to 350 Gb/in2. For future high area density recording, device dimensions will be in the range of 20-50 nm, or smaller. For such devices to continue operating with a similar SH-to-reader width aspect ratio (AR) as well as with a similar device MRR, the R.A (Resistance Area product) needs to be reduced by a factor of about 4 relative to today's devices. Usually dR/R drops as R.A is reduced and device noise increases as dimensions get smaller. These factors imply that it is very difficult to outperform or even to maintain current output amplitudes. Additionally, the inter-coupling field will be much larger when the use of MgO lowers R.A so that lapping control to a SH of 30-50 nm will become increasingly more difficult.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 6,512,661 (Louis) shows a GMR sensor including insulating bias layers surrounding the MR head. In U.S. Pat. No. 7,215,610, Sako et al. discuss digital output from a playing control system that is converted to analog output while U.S. Patent Application 2004/0156148 (Chang et al) discloses an MR head with longitudinal hard bias.